Digital Subscriber Lines (DSL) are a common technology for providing digital communication. In early DSL deployments, a broadband connection to a customer's premises was provided by a DSL connection extending over an existing twisted copper pair subscriber line between two DSL modems located in the exchange (Central Office in US terminology) and the customer's premises respectively. As DSL technology developed, a large portion of each DSL connection was replaced with optical fiber by extending the fiber between the exchange and another element in the Access Network, such as the street cabinet (for Fiber-To-The-Node deployments) or Drop Point Unit (DPU) (for Fiber-To-The-Dp deployments). The broadband connection then consisted of a fiber connection from the exchange to this intermediate point, followed by a DSL over copper connection to the customer's premises. In some deployments, the broadband connection consisted of a fiber connection all the way to the customer's premises (in Fiber-To-The-Premises deployments).
DSL networks have been upgraded on an ad hoc basis, such that some subscriber lines have been upgraded to any one of the Fiber-to-the-X scenarios above. DSL connections between elements of the DSL network are grouped together in bundles, which now typically comprise a mixture of copper, Fiber-To-The-Node, or Fiber-To-The-Dp connections in a single bundle. This can create an interference (crosstalk) issue between the different types of connections in the bundle. In an example scenario, a bundle includes a first subscriber line consisting of a copper connection between the exchange and the customer's premises, and a second subscriber line consisting of a copper connection from the street cabinet to the customer's premises. In this scenario, a signal on the first subscriber line is transmitted along the copper connection from the exchange, and a signal on the second subscriber line is transmitted along the copper connection from the street cabinet. Accordingly, the transmit power levels from the street cabinet must be reduced to take into account the level of attenuation of the signal on the first subscriber line. If not (i.e. if the two signals were transmitted at the same power levels), the signal on the second subscriber line would cause a large amount of crosstalk on the first subscriber line.
Network Operators therefore define the transmit power levels for transmissions in the DSL network. This is in the form of a “spectral mask”, which defines the power levels for each frequency in the transmission. These spectral masks are defined by the standard bodies for each form of DSL connection (for example, Recommendation ITU-T G.992.5 for ADSL2+ and Recommendation ITU-T G.9700 for G.fast), and are applied to all subscriber lines in the DSL network.
The spectral mask will now be described in more detail, with particular reference to the spectral mask for G.fast connections as described in the G.9700 standard. The transmit Power Spectral Density (PSD) mask (“TxPSDM”) is the maximum possible PSD of a transmit signal at a particular frequency for a G.fast transceiver. The TxPSDM is a mixture of the limit PSD mask (“LPM”), which specifies the absolute maximum limit of the TxPSDM, whilst the sub-carrier mask (“SM”), PSD shaping mask (“PSM”), notching mask (“NM”) and low-frequency edge stop-band mask (“LESM”), shape and reduce the levels of the LPM to produce the TxPSDM.
The notching masks are capable of “notching out” specific frequency bands by reducing the power levels for those frequency bands to a negligible amount. This is used to ensure that the transmission does not interfere with other Radio Frequency (RF) services, such as local FM, DAB, aeronautical, maritime or military radio. This is particularly relevant for G.fast transmissions as the higher frequencies (compared to other DSL technologies) suffer from a greater level of electromagnetic leakage and share the same RF spectrum with several RF services.
The Network Operator therefore selects a single TxPSDM for all connections of a particular form (e.g. a TxPSDM for all G.fast connections in the DSL network, and a TxPSDM for all VDSL2 connections in the DSL network). When a new connection is set up, the particular TxPSDM for that form of connection is then used for all transmissions on that connection.
The present inventors have realized that the existing methods of allocating power levels to transmission in DSL networks can be improved.